1. Field of the Invention
The present invention relates to a disk-shaped information recording medium having concentric or spiral tracks on which servo signals for detecting tracking error are recorded in a discrete manner, and to a method and an apparatus for recording and/or reproducing information, utilizing such a recording medium.
2. Related Background Art
Optical disk apparatuses are commonly known as apparatus for recording and/or reproducing information on a disk-shaped information recording medium. In such apparatus, since the pitch of information tracks is as small as 1.5 to 1.6 .mu.m, there is generally conducted a tracking servo for causing a light spot to track an information track, in which an objective lens working as light collecting means is moved by the deviation of the light spot from the information track, detected by optical means. Such a detection of the deviation is, for example, achieved by a discrete time method, usually called a sampling servo method. In such a method, as disclosed in the Japanese Patent Laid-open Application No. 59-3728, and as shown in FIG. 1, a front (or preceding) wobble pit 2 and a rear (or succeeding) wobble pit 4 are positioned with a deviation of about 1/4 track pitch on both sides of an imaginary (or virtual) track 1, and the deviation from the imaginary track is detected on the basis of the optical modulation by such wobble pits. More specifically, the optical disk is rotated at a constant angular velocity, and, if the light spot passes over the pits on a trace line A deviated toward the periphery of the disk, on the imaginary track 1, and on a trace line B deviated toward the center of the disk as shown in FIG. 2, the light spot is modulated by the wobble pits as indicated by curves e, f and g in FIG. 2, respectively. Thus, when the light spot passes on the trace line A, it is closer to the front wobble pit 2 and farther from the rear wobble pit 4 so that an optical modulation by the front wobble pit 2 is larger and an optical modulation by the rear wobble pit 4 is smaller. The situation is reversed when the light spot passes on the trace line B. On the other hand, when the light spot passes on the imaginary track 1, optical modulations by the front and rear wobble pits 2, 4 are equal.
In this method, a clock pit 3 is used for generating a reference clock signal for extracting a tracking error signal. The wobble pit train consisting of the front wobble pit 2, clock pit 3 and rear wobble pit 4 is positioned at a constant angular interval in the circumferential direction of the optical disk. Accordingly, when the optical disk is rotated at a constant angular velocity, the light spot periodically passes the wobble pit train. A signal indicating the peak of modulation by the clock pit 3 is generated and a reference clock signal h shown in FIG. 2 is generated by a phase-locked loop (PLL) circuit. The front wobble pit 2 and the rear wobble pit 4 are so positioned that the modulation signals thereof are separated from the clock signal by a multiple of the period of the reference clock signal in the rotation of the optical disk with a constant angular velocity.
The tracking error signal is obtained by sample-holding the modulation signal based on the front wobble pit 2 at a time t.sub.1 (at the setup of the reference clock signal), shown in FIG. 2, then sample-holding the modulation signal based on the rear wobble pit 4 at a time t.sub.2, and subtracting the latter signal from the former signal. Thus, when the light spot passes on the imaginary track, the result of the subtraction is zero, because the modulation signals resulting from both wobble pits are mutually equal. When the light spot is on the trace line A, the subtraction provides a positive result as the modulation by the front wobble pit 2 is larger than that by the rear wobble pit 4, and when the light spot is on the trace line B, the subtraction provides a negative result as the modulation by the front wobble pit 2 is smaller than that by the rear wobble pit 4. The tracking error signal is detected from the wobble pit trains at a substantially constant interval, namely in a discrete time manner.
The modulation signal resulting from the wobble pit train is generally repeated at a constant interval over the entire surface of the optical disk. This is particularly effective in a seeking operation. In the case of radially moving the light spot and effecting the tracking servo operation on a desired track, if the modulation signals resulting from the wobble pit trains do not appear periodically before and after the movement, the locking function of the PLL circuit is again required so as to generate the reference clock signal for extracting the tracking error signal, so that a prompt capture operation of tracking servo is difficult.
In such an information record/reproducing apparatus employing the sampling servo method, it is conceivable to utilize the reference clock signal for extracting the tracking error signal, also as the reference clock signal for the digital information to be recorded between two wobble pit trains. Thus, the digital information is recorded by modulating the beam of a semiconductor laser in synchronization with the reference clock signal h shown in FIG. 2, thereby recording marks in the form of pits (in the case, of write once read many type), magnetic domains (in the case of photo-electro-magnetic type) or the like in the information recording film between two wobble pit trains. The reproduction of information can also be achieved by identifying the presence or absence of said marks, by inspecting the signal obtained by photoelectric conversion of the light reflected from the disk at timings synchronized with said reference clock signal h shown in FIG. 2.
However, such utilization of the same reference clock signal both for tracking error detection and for recording and/or reproduction results in a reduced efficiency of utilization of the surface area of the optical disk, when it is rotated at a constant angular velocity. An important limitation on recording or reproduction is posed by the size of the light spot. In an ordinary optical disk apparatus, the half-peak width of the light spot is about 1.5 .mu.m, with a semiconductor laser having a wavelength of about 830 nm and an objective lens with a numerical aperture of 0.53. For securely achieving recording or reproduction with such a light spot, the minimum distance between the marks is about 1.5 .mu.m. In an optical disk rotating at a constant angular velocity, the linear speed of a track is lowest at the innermost track and becomes larger as the position approaches the external periphery. Consequently, if the revolution of the optical disk and the reference clock signal are so determined as to obtain a minimum distance of 1.5 .mu.m between the marks on the innermost track in the recording area, such a minimum distance on the outer tracks becomes larger than 1.5 .mu.m, so that the surface area of the optical disk is wasted.
In order to improve the efficiency of utilization of the optical disk, there has been proposed a modified constant angular velocity (MCAV) method, in which the disk is radially divided into plural zones, and the frequency of the reference clock signal is selected higher in the outer zones.
However the MCAV method cannot be applied to the sampling servo method explained above, because, if the record/reproducing head makes an erroneous access to a wrong zone in the access to a desired track, the tracking cannot be locked owing to the difference in frequency of the reference clock signal, so that the operation thereafter is disabled.